Small signal amplifier particularly for flow meter monitoring

ABSTRACT

A monitoring system has a small signal amplifier to amplify the low millivolt signals of a remotely located flowmeter. A pair of transistors are mutually parallel connected in emitter follower configuration by a variable bias potentiometer to the meter through three terminals. Each transistor supplies an input to an analog comparator with resistive and capacitive feedback to provide common mode rejection with a single-ended output signal. An isolating transistor in common collector configuration is connected to the output of the analog comparator for impedance transformation. The transistors are selectively connected to receive either a pair of balanced incoming signals or an unbalanced signal. A reference signal is applied as an input to one of the parallel connected transistors in unbalanced operation.

United States Patent Tur [54] SMALL SIGNAL AMPLIFIER PARTICULARLY FORFLOW METER MONITORING [72] Inventor: Charles J. Tur, Mequon, -Wis.

[73] Assignee: A. 0. Smith Corporation, Milwaukee, Wis.

22 Filed: May 21,1970

211 Appl. No.: 39,352

[1 1 3,681,984 [4 1 Aug. 8, 1972 5/1967 Hayes ..330/15 3/1969 Jones..330/30D Primary Examiner-Charles A. Ruehl Attorney-Andrus, Sceales,Starke and Sawall ABSTRACT A monitoring system has a small signalamplifier to amplify the low millivolt signals of a remotely locatedflowmeter. A pair of transistors are mutually parallel connected inemitter follower configuration by a variable bias potentiometer to themeter through three terminals. Each transistor supplies an input to ananalog comparator with resistive and capacitive feedback to providecommon mode rejection with a singleended output signal. An isolatingtransistor in common collector configuration is connected to the outputof the analog comparator for impedance transformation.

The transistors are selectively connected to receive either a pair ofbalanced incoming signals or an unbalanced signal. A reference signal isapplied as an input to one of the parallel connected transistors inunbalanced operation.

14 Claims, 2 Drawing Figures J Logic Gate Counter PATENTEDAUG 8 I972LgtEmcm @231 Lgcsou .QOUbIllQi.

INVENTOR. CHARLES J. TUR ZZ/Z/4/% Attorneys SMALL SIGNAL AMPLIFIERPARTICULARLY FOR FLOW METER MONITORING BACKGROUND OF THE INVENTION Thisinvention relates to a small Signal amplifier capable of receivingbalanced or unbalanced low millivolt signals transmitted from a remotelylocated flowmeter.

Analog comparators have been used to receive floating or differentialinputs in balanced operation to provide an amplified single-ended orground referenced output signal. Balanced floating input signals couldbe provided by transmissions received from a remotely located turbineflowmeter producing an AC. output proportional to the speed of theturbine which is also proportional to the volumetric fluid flow throughthe fluid line, as shown in the patent to Erickson, US Pat. No.3,518,881, issued July 7, 1970 and assigned to a common assignee.

parator. If not externally compensated, offset bias cur,

rent and voltage is drawn from one floating input to cause output errorin the comparative difference between signals.

Analog comparators capable of receiving balanced floating input signalshave not been employed to receive an unbalanced signal from a remotelylocated meter because of the comparing nature of the comparatorsoperation. Such an unbalanced input signal could be provided bytransmissions received from a turbine flowmeter transducer whichgenerates a pulse output proportional to volumetric flow through a fluidline as shown in the patent to I-Ioule, entitled Signal TransmittingCircuit for Remote Sensing of Petroleum Flow, US. Pat. No. 3,319,255,issued May 9, 1967 and owned by a common assignee. The analogcomparators low input impedance has also limited its use.

SUMMARY OF THE INVENTION This invention relates to a small signalamplifier and particularly to such an amplifier capable of receivingbalanced or unbalanced low millivolt signals transmitted from a remotelylocated flowmeter.

A pair of amplifying transistors are parallel .connected in commoncircuit with a variable bias potentiometer to supply a pair of floatinginput signals to an analog comparator. The pair of transistors areselected for low ofis et input voltage and for high current gaincompatible with the capabilities of the analog comparator.

The pair of amplifying transistor circuits further provide high inputimpedance and thereby reduce or essentially eliminate loading of thereceived transmitted signals due to current demand and further providelow output impedance which facilitates coupling to the analogcomparator.

A variable bias potentiometer is connected in common with the pair ofamplifying transistors and regulates the relative current flow betweeneach transistor. Ofiset bias current and voltage, inherently required byanalog comparators, is therefore provided by the un balanced steadystate current flow between transistors when connected as floating inputsto the comparator. Thus, by compensating the floating inputs through thevariable bias potentiometer, offset bias current and voltage required bythe comparator will not be supplied by the received signals and accuratecomparison will occur between the two received amplified signals foroptimum common mode rejection.

A further aspect of the invention for optimum operation provides-aresistive and capacitive input circuit to each parallel connectedamplifying transistor to supply high input impedance and provide anupper cut off frequency for the analog comparator. In like manner, aZener diode is employed in the input circuit to act-as a voltage clampand supply input overdrive protection.

A reference signalsource is selectively connected as part of the inputcircuit when. receiving an unbalanced transmitted signal. A receivedunbalanced signal will be applied to one of the parallel connectedtransistors in accordance with the polarity of the received signalrelative to the internal biasing of the analog comparator while areference signal is supplied to the opposite parallel connectedtransistor. Thus, two floating inputs will be supplied to the analogcomparator whose output will depend on the magnitude of the suppliedreference signal.

In another aspect of the invention for optimum operation, a couplingcapacitor is connected in common with the pair of amplifying transistorsto short circuit the analog comparators input when receiving highfrequency noise, thereby protecting the comparator.

The analog comparator also employs a parallel connected resistive andcapacitive feedback circuit toprovide direct current stability withhysteresis to suppress noise while allowing high speed feedback responsefor input level changes. In particular, a feedback capacitor is providedwhich allows rapid feedback at very low frequencies.

A further aspect of the invention for optimum operation employs a commoncollector amplifying transistor to receive the single-ended analogcomparator output signal and to supply a low impedance output signalcompatible with electrical equipment used for measurement, control andthe like.

BRIEF DESCRIPTION OF THE DRAWING The drawing furnished herewithillustrates the best mode presently contemplated by the inventor andclearly discloses the above advantages and features as well as othersthat will be readily understood from the 3 detailed description thereof.

In the drawing:

FIG. 1 is a schematic circuit of a monitoring system DESCRIPTION OF THEPREFERRED ILLUSTRATED EMBODIMENT A preferred construction of a smallsignal amplifier 1, shown scehmatically in FIGS. 1 and 2, connects alogic gate 2 and recording meter 3 to a transmission line 4 and turbineflowmeter 5.

Turbine flowmeter 5 has an annular vane type rotor 5a within a fluidline 6 which rotates when subjected to fluid pressure flow. An A.C.generator or transducer 7 is coupled to turbine flowmeter 5 andmagnetically responds to rotations of rotor 5a. Such rotations providean alternating current output proportionalto the speed of rotor 5a andtherefore proportional to the volumetric fluid flow through line 6; forexample, as shown in the patent to Erickson, US. Pat. No. 3,518,881.,issued on July 7, 1970 and assigned to a common assignee.

As shown in FIG. 1, balanced signals produced by turbine flowmeter 5 aretransmitted on conducting wires 8 and 9 through shielded transmissionline 4 to the small signal amplifier l. The shielding of transmissionline 4 is grounded by a wire 10 to turbine flowmeter 5.

Transmission line 4 may also contain other wires, not shown, and may beconnected to monitor several flowmeters simultaneously.

Small signal amplifier 1 is constructed in accordance with the teachingof the present invention to receive low millivolt signals with amultiple input circuit 11 connected to a pair. of parallel connectedtransistors 12 and 13. The floating inputs from conducting wires 8 and 9are connected to transistors 12 and 13 as follows.

The pair of parallel connected amplifying transistors 12 and 13 areconnected in emitter follower configuration to receive the floatinginputs from input circuit 1 l. Transistors 12 and 13 are chosen withnearly identical characteristics as practical or economical. However, itis not mandatory that their characteristics be exact for accurateoperation. The operation of transistor 12 and associated circuitry willbe described in detail only since transistor 13 operates in like mannerand the corresponding circuitry of transistor 13 will be designated byprime numbers.

Transistor 12 is shown as a PNP type with a base 14, emitter 15 andcollector 16. Base 14 is connected to input wire 8 through a seriesconnected resistor 17 and to ground through a parallel connected Zenerdiode 18, resistor 19 and capacitor 20. Zener diode 18 provides inputoverdrive protection to the base-emitter junction of transistor 12 andsubsequently to an analog comparator 21. Resistor 19 and capacitor 20establish an upper cut off frequency while resistors 17 and 19 establisha high input impedance for small signal amplifierv l.

A positive voltage source is schematically shown by the legend V+ andconnected to emitters 15 and 15 through emitter resistors 22 and 22,respectively, and

a common variable potentiometer 23. Specifically, variable potentiometer23 connects the common ends of resistors 22 and 22' to voltage source V+through the tap 24 of potentiometer 23. The position of tap 24 inverselyvaries the resistance in the respecfive emitter circuits of transistors12 and 13. Collectors 16 and 16' are mutually connected to the negativeside of the voltage source schematically shown by the legend V.

In operation, a current path is established through transistor 12.fromvoltage source V+ through tap 24, potentiometer 23, resistor 22, emitter15, and collector 16 to voltage source V.

Transistor 13 operates in similar manner and the amount of currentflowing in each branch is basically dependent upon the bias setting ofvariable potentiometer 23 and the input voltage signalsupplied to bases14 and 14'. A pair of floating outputs from transistors 12 and 13 aretaken from emitters l5 and 15 in accordance with known emitter followerdesign and applied as floating inputs to analog comparator 21.

Analog comparator 21 is designed to compare two floating input signalsand provide a single-ended output with common mode rejection. Comparator21 includes a high gain amplifier of customary design well known tothose skilled in the art and is therefore shown diagrammatically. Inaccordance with a further teaching of this invention, a parallelconnected resistor 25 and capacitor 26 is connected in a feedbackcircuit to analog comparator 21. Applicant determined that the systemwithout feedback encountered noise related hysteresis and inadequateresponse to input level changes. The R.C. feedback path provides directcurrent stability with hysteresis to suppress noise and also provides ahigh speed feedback response for input level changes, even at very lowfrequencies.

Offset bias current and voltage is inherently required by analogcomparator 21 due to natural internal circuit inbalance. The parallelconnected transistors have the relativesteady state currents flowing inemitters 15 and 15 set by adjusting variable potentiometer 23 which inturn adjusts the steady-state floating input levels to comparator 21.This steady state compensation supplied by the pair of transistors 12and 13 will prevent analog comparator 21 from drawing oflset biascurrent and voltage from the transmitted floating signals andallowaccurate comparison.

A coupling capacitor 21a is connected across emitters 15 and 15' andthus across the two floating inputs to analog comparator 21 to shortcircuit emitters 15 and 15 when high frequency noise is received. Theshort circuit function of capacitor 21a thereby protects the analogcomparator 21 from possible damaging and disrupting high frequencynoise.

The single-ended output of analog comparator 21 is supplied to anisolating or bufier transistor 27 illustrated as a grounded collectorPNP type transistor. Transistor 27 provides a low impedance outputsignal compatible with most integrated logic circuits used for 1metering or control. Specifically, a single-ended output signal is takenfrom emitter 28 and supplied to logic gate 2 and counter 3. The outputof logic gate 2 could be utilized in various control and indicatingoperations,

such as indicating the rate of flow being sensed.

An unbalanced signal transmission is shown in FIG. 2 received andamplified by small signal amplifier 1. The unbalanced signal istransmitted on wire 29 from pulse transmitter 30 which converts analternating current signal supplied by generator or transducer 7 intopulses indicative of the speed of rotor 5a and therefore indicative ofvolumetric fluid flow through line 6. Pulse transmitting circuits wellknown to those skilled in the art may be used, such as that shown in thepatent to Houle, entitled Signal Transmitting Circuit for Remote Sensingof Petroleum Flow, U.S. Pat. No. 3,319,255, issued May 9, 1967 and ownedby a common assignee.

In accordance with normal operating procedures, the shielding oftransmission line 4 is grounded by connecting wire 31 to pulsetransmitter 30. The unbalanced signal received on wire 29 is supplied asan input to small signal amplifier l to either transistor 12 or 13,depending upon the signal polarity of the incoming unbalanced signalrelative to the biasing within analog comparator 21. A reference signalsupplied by reference source 32 is supplied to the opposite transistor12 or 13.

Reference signal source 32 is any suitable signal source. In FIG. 2, aDC voltage source is diagrammatically illustrated as V+ and connectedthrough wire 33 as an input to transistor 13. For optimum constructionand operation, a variable voltage divider is employed to vary themagnitude of the reference signal and is shown by the connection ofresistor 34 and variable resistor 35 in voltage divider arrangement.Specifically, resistor 34 is connected between voltage source V+andoutput point 36 while resistor 35 is connected between ground and outputpoint 36. Adjustment of the reference signal supplied from source 32produces a minimum voltage level at emitter which supplies an input toanalog comparator 21 to be compared to the input supplied by theunbalanced signal at emitter 15. A minumum input comparing levelsupplied by source 32 is used to eliminate unwanted noise associatedwith the unbalanced signal in a manner analogous to balanced operationwhich allows optimum signal detection.

The present invention provides a highly satisfactory and reliable smallsignal amplifier capable of receiving low millivolt signals receivedfrom remotely located turbine flowmeters and employing common moderejection in balanced operation while permitting convenient adaptationfor unbalanced operation with essentially identical amplificationcircuitry.

Various modes of carrying out the invention are contemplated as beingwithin the scope of the following claims particularly pointing out anddistinctly claiming the subject matter which is regarded as theinvention.

Iclaim:

1. A small signal amplifying circuit for differentially amplifying apair of low voltage signals with rejection of common mode voltagesignals appearing on said low voltage signals, comprising a signalsource means establishing said pair of low voltage signals and includinga transducer for establishing at least one of said low voltage signalsas a variable signal to be detected,

a first and a second high input impedance means connected to the sourcemeans and each receiving one of said pair of low voltage signals and thecorresponding common mode voltage signal,

a first and a second amplifying transistor each connected in an openloop amplifying circuit to said first and second high input impedancemeans respectively to receive said two input voltage signals and saidcommon mode voltage signals and to establish a pair of output signalsincluding both said input voltage signals and said common mode voltagesignals received from said first and second impedance means, and

an analog comparator means responsive to the difference voltage andhaving a first input means connected to said first amplifying transistorand a second input means connected to said second amplifying transistorfor combining said pair of output signals supplied by said first andsecond amplifying transistors and constituting the means for rejectingsaid common mode voltages with said common mode voltages rejected at thefirst and second input means and providing a differential comparisonbetween said pair of output signals to supply a single-ended output.

2. The small signal amplifying circuit of claim 1, including anadjustable potentiometer connected to a bias voltage source andconnected in common to said first and second amplifying transistors toinversely vary the bias of said first and second amplifying transistorsand provide ofi'set bias current and voltage to said analog comparatormeans to allow accurate common mode rejection.

3. The circuit of claim 1 wherein said signal source includes,

a reference signal means connected to the other of said first and secondhigh input impedance means to supply a reference signal from that of thetransducer as the second low voltage signal in unbalanced operation.

4. The circuit of claim 1 including,

a flow metering means and said transducer means connected to saidmetering means to establish an output signal proportional to flow, and

connecting means to selectively connect said transducer means to atleast one of said first and second high input impedance means.

5. The circuit of claim 4, wherein said connecting means is connected toboth of said first and second high input impedance means to supply apair of signals indicative of flow rate in balanced operation.

6. The circuit of claim 4 wherein,

said signal source means includes a reference signal means selectivelyconnected to one of said first and second high input impedance means tosupply a reference signal, said connecting means selectively connectedto the opposite of said first and second high input impedance means tosupply a signal indicative of flow rate in unbalanced operation.

7. The circuit of claim 6, wherein said reference signal means includesa voltage source connected through a variable resistive voltage dividerto supply a reference signal.

8. The circuit of claim 1, wherein said first and second high inputimpedance means each includes a resistor serially connected to aparallel connected resistor, capacitor and Zener diode to supply highinput impedance, establish an upper cutoff frequency, and provideoverdrive protection for said small signal amplifying circuit.

9. The circuit of claim 1, wherein said analog comparator means employsa parallel connected resistive and capacitive feedback circuit toprovide direct current stability with hysteresis to suppress noise andallow high speed feedback response to signal level changes.

.- 10. The circuit of claim 1 including a coupling capacitor connectedto said first and second input means of said analog comparator means toshort circuit said first and second input means when receiving highfrequency noise.

11. The circuit of claim 1 and including an impedance transformationmeans connected to said analog comparator means and having a low outputimpedance to allow matching of said small signal amplifying circuit withsucceeding circuitry.

12. A small signal amplifying circuit for amplifying low voltage signalsreceived from a remotely located flowmeter, comprising a first and asecond input means including a resistor serially connected to a parallelconnected resistor, capacitor and Zener diode to supply high inputimpedance, establish an upper cutofl frequency, and provide overdriveprotection for said small signal amplifying circuit,

a first and a second amplifying transistor connected in common emitterconfiguration and connected to said first and second input meansrespectively to receive at least one low voltage signal and to establisha pair of output signals,

an analog comparator means including parallel connected resistive andcapacitive feedback circuit to provide direct current stability withhysteresis to suppress noise and allow high speed feedback response tosignal level changes, and including a first input means connected tosaid first amplifying transistor and a second input means connected tosaid second amplifying transistor to combine said pair of output signalssupplied by said first and second amplifying transistors with a couplingcapacitor connecting said first and second input means to short circuitwhen receiving high frequency noise, and

an adjustable potentiometer with a center tap connected to a biasvoltage source, said adjustable potentiometer connected through a pairof emitter resistors to said first and second amplifying transistors toinversely vary the bias of said first and second amplifying transistorsand provide ofi? set bias current and voltage to said analog comparatormeans to allow accurate common mode rejection.

13. The small signal amplifier of claim 12, including a transducer meansconnected to said flowmeter to establish an output signal indicative offlow rate,

connecting means connected to said transducer means and to said firstinput means to supply a flow indicative signal, and

a reference signal means including a voltage source selectivelyconnected through a variable resistive voltage divider to said secondinput means to supply a reference signal in unbalanced operation.

14. Thesmall signal amplifier of claim 12, including a transducer meansconnected to said flowmeter to establish a pair of output signalsindicative of flow rate, and I connecting means connected to saidtransducer means and to said first and second input means to supply apair of flow indicative signals in balanced operation.

1. A small signal amplifying circuit for differentially amplifying apair of low voltage signals with rejection of common mode voltagesignals appearing on said low voltage signals, comprising a signalsource means establishing said pair of low voltage signals and includinga transducer for establishing at least one of said low voltage signalsas a variable signal to be detected, a first and a second high inputimpedance means connected to the source means and each receiving one ofsaid pair of low voltage signals and the corresponding common modevoltage signal, a first and a second amplifying transistor eachconnected in an open loop amplifying circuit to said first and secondhigh input impedance means respectively to receive said two inputvoltage signals and said common mode voltage signals and to establish apair of output signals including both said input voltage signals andsaid common mode voltage signals received from said first and secondimpedance means, and an analog comparator means responsive to thedifference voltage and having a first input means connected to saidfirst amplifying transistor and a second input means connected to saidsecond amplifying transistor for combining said pair of output signalssupplied by said first and second amplifying transistors andconstituting the means for rejecting said common mode voltages with saidcommon mode voltages rejected at the first and second input means andproviding a differential comparison between said pair of output signalsto supply a single-ended output.
 2. The small signal amplifying circuitof claim 1, including an adjustable potentiometer connected to a biasvoltage source and connected in common to said first and secondamplifying transistors to inversely vary the bias of said first andsecond amplifying transistors and provide offset bias current andvoltage to said analog comparator means to allow accurate common moderejection.
 3. The circuit of claim 1 wherein said signal sourceincludes, a reference signal means connected to the other of said firstand second high input impedance means to supply a reference signal fromthat of the transducer as the second low voltage signal in unbalancedoperation.
 4. The circuit of claim 1 including, a flow metering meansand said transducer means connected to said metering means to establishan output signal proportional to flow, and connecting means toselectively connect said transducer means to at least one of said firstand second high input impedance means.
 5. The circuit of claim 4,wherein said connecting means is connected to both of said first andsecond high input impedance means to supply a pair of signals indicativeof flow rate in balanced operation.
 6. The circuit of claim 4 wherein,said signal source means includes a reference signal means selectivelyconnected to one of said first and second high input impedance means tosupply a reference signal, said connecting means selectively connectedto the opposite of said first and second high input impedance means tOsupply a signal indicative of flow rate in unbalanced operation.
 7. Thecircuit of claim 6, wherein said reference signal means includes avoltage source connected through a variable resistive voltage divider tosupply a reference signal.
 8. The circuit of claim 1, wherein said firstand second high input impedance means each includes a resistor seriallyconnected to a parallel connected resistor, capacitor and Zener diode tosupply high input impedance, establish an upper cutoff frequency, andprovide overdrive protection for said small signal amplifying circuit.9. The circuit of claim 1, wherein said analog comparator means employsa parallel connected resistive and capacitive feedback circuit toprovide direct current stability with hysteresis to suppress noise andallow high speed feedback response to signal level changes.
 10. Thecircuit of claim 1 including a coupling capacitor connected to saidfirst and second input means of said analog comparator means to shortcircuit said first and second input means when receiving high frequencynoise.
 11. The circuit of claim 1 and including an impedancetransformation means connected to said analog comparator means andhaving a low output impedance to allow matching of said small signalamplifying circuit with succeeding circuitry.
 12. A small signalamplifying circuit for amplifying low voltage signals received from aremotely located flowmeter, comprising a first and a second input meansincluding a resistor serially connected to a parallel connectedresistor, capacitor and Zener diode to supply high input impedance,establish an upper cutoff frequency, and provide overdrive protectionfor said small signal amplifying circuit, a first and a secondamplifying transistor connected in common emitter configuration andconnected to said first and second input means respectively to receiveat least one low voltage signal and to establish a pair of outputsignals, an analog comparator means including parallel connectedresistive and capacitive feedback circuit to provide direct currentstability with hysteresis to suppress noise and allow high speedfeedback response to signal level changes, and including a first inputmeans connected to said first amplifying transistor and a second inputmeans connected to said second amplifying transistor to combine saidpair of output signals supplied by said first and second amplifyingtransistors with a coupling capacitor connecting said first and secondinput means to short circuit when receiving high frequency noise, and anadjustable potentiometer with a center tap connected to a bias voltagesource, said adjustable potentiometer connected through a pair ofemitter resistors to said first and second amplifying transistors toinversely vary the bias of said first and second amplifying transistorsand provide offset bias current and voltage to said analog comparatormeans to allow accurate common mode rejection.
 13. The small signalamplifier of claim 12, including a transducer means connected to saidflowmeter to establish an output signal indicative of flow rate,connecting means connected to said transducer means and to said firstinput means to supply a flow indicative signal, and a reference signalmeans including a voltage source selectively connected through avariable resistive voltage divider to said second input means to supplya reference signal in unbalanced operation.
 14. The small signalamplifier of claim 12, including a transducer means connected to saidflowmeter to establish a pair of output signals indicative of flow rate,and connecting means connected to said transducer means and to saidfirst and second input means to supply a pair of flow indicative signalsin balanced operation.